LinkLocalAddressing fallback on DHCP failure

[tdphette@xxxxxxxxx (Thad Phetteplace)]

I'm currently working with a client that needs LinkLocalAddressing as a
fallback when DHCP resolution fails. Currently with systemd-networkd it
looks like you have both DHCP and LinkLocalAddressing on, but that means
you get a second 169.254.*.* address even when DHCP succeeds. We
investigated using avahi-autoipd, but it currently appears to work only
with dhclient, and we prefer to stick with networkd. Rather than add
networkd support to avahi, I think adding a LinkLocalAddressing 'failover'
option to networkd makes more sense.

My questions for my fellow developers are:

1) Am I missing some obvious solution that does not require writing new
code?
2) Would the systemd-devel community be open to this feature if I / my
client contributed it?
3) What should the syntax in the config file look like? I was thinking
something like this:

LinkLocalAddressing=fallback

Or if you wanted it only for a specific protocol, use 'ipv4fallback' or
ipv6fallback'. The other option would be to include an additional flag like
'LinkLocalAddressingFallback='. I'm open to whatever the community would
accept.

I appreciate your thoughts on this.

Thanks,

Thad Phetteplace
GLACI, Inc.
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